This invention pertains, generally, to centrifugal pumps used in the pulp industry to pump pulp slurries, and in particular to a centrifugal pump of such configuration as renders it capable of handling pulp slurry having up to approximately a fifteen percent consistency for long fibered pulp.
Traditionally, centrifugal pumps have been limited to handling pulp slurries of approximately four percent consistency. For higher consistencies, up to about eighteen percent, positive displacement pumps have been used. In the industry, recent developments by some manufacturers have pushed the limit for centrifugal pumps up to about a twelve percent consistency. Claims of the manufacturers notwithstanding, these pumps actually run in the ten to twelve percent consistency range, depending upon fiber length, and not the up to fifteen percent consistency alleged therefor.
It is very desirable to have a centrifugal pump which can run reliably at the fifteen percent consistency for long fibered pulp for the following reasons:
1. Bleach towers tend to channel at consistencies below fourteen percent, and thus shorten the retention time of the product. This becomes more important as the capacities increase and the bleach towers get larger in diameter. PA1 2. More steam is required to heat a more dilute slurry. PA1 3. Storage tanks are built, at some considerable expense, to store pulp, not water. PA1 4. Typically available infeeding filters discharge at fourteen to fifteen percent consistency. Dilution, then, to render the product acceptable to a receiving centrifugal pump is undesirable involving as it does another, expensive processing procedure. PA1 1. A larger diameter infeed section. PA1 2. A pulp inducer with an overfeeding capacity. PA1 3. Non-converging flow passages in the inducer and impeller. PA1 4. Large size flow channels in the inducer and impeller to minimize friction and to allow passage of tramp metal. PA1 1. Pulp at medium consistency (from ten to fifteen percent) contains a significant amount of air. This air is not harmful if it is evenly dispersed. But, the vortex will centrifuge air out of the suspension. Air will accumulate at the eye of the impeller and air-bind the pump. PA1 2. A feed screw pushes the pulp towards the inducer. In that the feed screw rotates in the same direction as the impeller, the rotating vortex tends to stop or impede the pulp flow in the feed screw. This is depicted in FIG. 1, herein. PA1 3. The vortex creates a low pressure zone at the eye of the impeller; i.e., the suction head of the pump is negative. PA1 4. The vortex consumes unnecessary power.
Centrifugal pumps designed to handle higher consistency (albeit not fifteen percent) pulps incorporate the following design features:
These centrifugal pumps, however, have serious defects. The overfeeding inducer recirculates some pulp into the feed area (i.e., regurgitation). The recirculated pulp has a large rotational velocity and causes a strong vortex ahead of the inducer. The vortex increases in intensity as it is drawn towards the eye of the impeller (i.e., as a contracting vortex). This vortex has several disadvantages:
The foregoing details the limitations and disadvantages known to exist in the prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the aforesaid limitations and disadvantages. Accordingly, a suitable alternative, embodied in a novel pulp slurry-handling, centrifugal pump, is set forth herein, the same having features more fully disclosed hereinafter.